Thermal Conductivity vs. Hardness

[aspearin1]

I'm looking for a material that has a thermal conductivity higher than that of aluminum with a hardness comparable to cold-rolled steel. Can aluminum be treated to reach such rigidity? Are there any copper alloys that may work? The material will go into a work piece that periodically sustains 1/2 ton of pressure (for 1 sec.) at 800 deg F. Any recommended resources?

 

[NickE]

1000lbf over what amount of cross sectional area?.. Is hardness your main physical requirement (wear resistance?) or is it compressive strength and thermal conductivity...

Try doing a search on Mat-Web using the property requirements..

I think that a CuBe etc. alloy might give you what you need...
nick

 

[unclesyd]

Aspearin1,
You might be able to use one of the Ampco metals. I don't have their brochure anymore so I can’t look up the physical properties of their materials..

http://www.ampcometals.com

http://www.copper.org

This site will give you the physicals of the families of materials.

 

[aspearin1]

Thank you for the replies. The contact surface area is very small. It consists of 2 parallel knife edges about .010" wide and 4" long (in a rectangular perimeter). Currently steel is the metal of choice. Compression strength of these knife edges are of primary concern. It also recently came to my attention that our current parts have annealed over extended heating and cooling under these conditions. High heat with ambient machine cool. I know little about metallurgy, so my question is broad. Is there a predictive model that can tell me when to change a part based on an initial hardness and so many heating/cooling cycles? Also, is it possible to re-harden a part that has been annealed in such a manner without changing the physical properties of the piece? Would a re-hardened part follow the same annealing patterns as the original part at the same target hardness? So much to learn.

 

[unclesyd]

If you are using cold rolled carbon stell now why not switch to a hot work die steel that should work hundreds of times longer than what your are now using.

Your part is probably not annealing as its not very hard to begin with. It probably just deforming from use as C/S doesn't have much strength at your temperature.

Unless your cold roll steel is hardenable by heat treating the answer is that nothing can be done to it. It could possibly be case hardened to help out, but I would go with my suggestion to change steels.

 

[Maui]

There are three things that you need to relate in order for an appropriate response to be posted. Do the parts in question wear out, chip out, or permanently deform? If they wear out then you will need a material with greater wear resistance than you are currently using. If the parts chip out, then toughness is the main issue and you need a material with higher toughness. If the parts permanently deform, then they are not hard enough. You stated that you believe the parts are tempering back due to the thermal cycling that they are exposed to during use. If this is the source of your problem, then a material such as a hot work die steel, as suggested by unclesyd, would be an appropriate substitute. H13 may be a good choice because it can tolerate exposures as high as 1000 F for extended periods of time without a significant reduction in hardness. Since your temperature extreme is only 800 F, this would probably work well. But keep in mind that toughness and wear resistance also play key roles in determining how well a material will perform in a given application. You need to consider these as well in making an appropriate choice for a substitute material.


Maui

 

[aspearin1]

Great input, and very appreciated. I am in agreement that the cold-rolled steel is not a good choice for this tool. I will look into the harder options,(H13). Can I get some insight from a machinist point of view? How machineable might the H13 be for small radi (.030&quot and a flatness of +/- .001"? I believe machinability was the main reason for choosing the cold-rolled steel. Currently, they are not being case hardened, and yes, they are deforming under pressure after prolonged use. Chipping is not an issue. I still believe the piece is undergoing some tempering, though. A new piece gives me a rockwell B of 90, while an older piece is about 60. No, they are not from the same stock, but I'd be surprised of such a hardness swing. If there is tempering going on, can I speed the cooling rate of this piece to extend the life of its hardness? And again, can it be re-hardened without actually damaging the piece? In the short term, I will experiment with some case hardened pieces and look into H13. Thanks again for the input.

 

[Maui]

Cold rolled steel obtains its hardness from mechanical deformation, not thermal treatment. By subjecting it to the thermal cycles that you described you are effectively stress relieving it. A hot work die steel such as H13 obtains its hardness from thermal treatment, not mechanical deformation.

The machinability of H13 is a strong function of its hardness. In the annealed condition it is readily machined using standard practices including milling, drilling, and single point turning. In the heat treated condition it is much more difficult to machine due to its increased hardness. The maximum attainable hardness for this through-hardening grade of steel is about 53 Rockwell C, but in application it is normally used in the 40-48 HRC range. It can be surface finished to a high tolerance especially in the hardened condition, and provides a good balance of toughness, heat check resistance, and high temperature strength. It possesses only moderate wear resistance. Does this answer your question?


Maui

 

[aspearin1]

maui,
Very good. I think I'm close to understanding it. So if I chose to rework the cold-rolled piece, shot peening would be a better option than case hardening. Correct? So is case hardening simply impossible for cold-rolled steel, or just not very effective? Will the surface not absorb carbon during treatment? Your input has been very valuable for me. -aspearin1

 

[Maui]

Shot peening the part or case hardening it will produce a hardened surface layer that is relatively thin. These techniques are typically used for applications where surface wear is a concern. Since your problem seems to be permanent deformation under load and not surface wear, I suspect that neither will help you very much in this particular application.

I would recommend that you consider a material substitution with a through-hardening grade of steel that would be appropriate for this application. Keep in mind the precautions that I mentioned in my prior posts regarding toughness and wear resistance.


Maui

 

[NickE]

case hardening such as nitriding/carburizing will increase your surface hardness only, As your parts are deforming after time at load and temp then hot strength is going to be your important design consideration. I'll have to agree with H13 as a good choice. After heat treating a good machine shop (Tool/Die) should be able to use surface/jig grinders to get you +/- .0005" or better with ease. The small radi that you mention shouldn't bee too much trouble. I would do some research on the heat treat parameters so that you can specify exactly what you want, at least a double temper w/ a cryo step should give you good dimensional stability, esp w/ a 1200F tempering temp. I would spec final HRc 46-48. This is much much harder than the 90HRb of your current parts. An estimated UTS for this steel is roughly 200ksi, at least twice as strong as most CR low carbon/low alloy.

This will definately cost more but you will have several benifits:

1. the H13 part will retain their properties over much longer times... possibly as long as you could ever want..
2. Results will be much more consistant.


nick

 

[unclesyd]

Your are going to have to use a through hardened material.
Are these components insert blades?
If they are an insert, you might want to try feeler gauge stock.

http://www.precisionbrand.com/default.asp

It is already heat treated to 50-55 Rc and relatively cheap.

Almost any thing you do ti .010" material is going to warp it.

 

[unclesyd]

This is a repeat, but complete, of the last post.

Your are going to have to use a through hardened material.
Are these components insert blades?
If they are an insert, you might want to try feeler gauge stock.

http://www.precisionbrand.com

It is already heat treated to 50-55 Rc and relatively cheap.

There is another material, reed valve springs, that is also very hard and tough.

The industrial knife and blade people alos make blades in your size range. We used a .012 3/4" x 6" blades in some very rough duty.

http://www.knives-ez.com/knives/0005051

This is a good starting point.

 

[JimMetalsCeramics]

You might try aluminum oxide dispersion strengthened copper (Glidcop) or SiC-reinforced aluminum. If you are able to use a ceramic, aluminum nitride.

 

[aspearin1]

Thanks for the input. To overcome my thermal conductivity problem, I am testing a steel piece with an aluminum inlay. Hopefully this will give me the strength on my knife edges (steel) and give me the thermal conductivity on my other surfaces. I will also look into the through hardened steels for future pieces.